Pressing of articles from metal powder



.5. 31, 1950 R. E. RICE 2,495,823

' PRESSING 0F ARTICLES FROM METAL POWDER Filed Dec. 2, 1946 fiozzfowder zrazzpowder YIIIIIIIIIIIIIII/ Yllllllllllllll Patented Jan. 31, i950 UNITED S'I'ES NT OFFICE 2,495,823 PRESSING 0F ARTICLES FROM MET WDER Application December 2, 1946, Serial No. 713,463

This invention relates to the art of powder metallurgy, and particularly to the preventionof excessive wear of die surfaces during the pressing of blanks made from powdered metal. It is particularly valuable in fabricating steel parts of high quality physical properties by powder metallurgy processes in the course of which the powdered starting materials are compressed, sintered, recompressed, and resintered.

In the art of powder metallurgy it is generally necessary when making steel compacts of high quality physical properties to subject the powdered starting material to a sequence of steps including (1) compressing the powder into a porous briquette, (2) sintering the briquette, (3) repressing the sintered briquette to increase its density, and (4) resintering the briquette. The powdered starting material for making steelarticles usually contains iron powder mixed with powdered graphite or other carbonaceous material and a lubricant. Such a powder mixture is usually compacted into briquettes at pressures of 40 tons per square inch or less, and very little abrasion of the die surfaces takes place during this first pressing step, since the iron is much softer than the die steel. I have for example found in making briquettes from electrolytic iron powder containing 1.0% powdered stearic acid lubricant in dies made of high chrome high carbon steel and using a pressure of 27 tons per square inch, that the wear on the die surfaces was less than .0001 inch after the pressing of about 35,000 briquettes. Addition of powdered graphite to such a powder mixture does not increase the die wear; powdered graphite is well known for its lubricating properties. However,

during the first sintering operation the graphite.

becomes dissolved in the iron, and upon cooling the briquette may have a pearlitic or other structure which has as a constituent iron carbide in finely divided form. Therefore, the surfaces of briquettes made acc0rding to conventional methods contain many exposed particles of this hard, abrasive iron carbide. When such a briquette is repressed in a die, these abrasive particles come into forceful contact with the die surfaces, and considerable wearing of the die takes place during recompression and subsequent ejection of the briquette, iron carbide being as hard, or harder, than the best die steels. Since the repressing operation takes place at pressures of 60 tons per square inch, or over, this wearing is very serious indeed, shortening the life of the die and increasing the cost of making the parts.

It is one of the principal objects of the inven- 3 Claims. (Cl. 75-22) tion to prevent excessive wearing of die surfaces during repressing operations in the making of steel compacts. Other objects are to provide a method of pressing steel articles from powdered material employing the steps of compressing, sintering, repressing and resintering without incurring substantial die wear, while avoiding additional steps or employing other than the usual powder and die materials; and to provide such a technique which can be carried out without essentially increasing the cost or duration of the process. A

In accordance with one aspect of my invention, these objects are accomplished by removing carbon from the surface of steel compacts at an intermediate stage of manufacture, so that when the compacts are repressed in a die, abrasion of the die surfaces is minimized.

In another aspect, the invention contemplates the restoration of carbon to the surfaces of steel compacts, following operations of surface decarburizing heat-treatments and repressing.

In some of its more specific aspects, the invention reduces excessive die wear by substantially completely decarburizing a thin layer at the surface of the compact'during the latter part of the first sintering operation, so that during the repressing operation that portion of the compact which is in contact with the die is soft iron free from hard iron carbide particles rendering the wear and abrasion on the die very small, whereupon, that is after the repressing operation, the desired amount of carbon is during the resintering operation restored to the surface of the compact so that the final compact may be hardened, as by quenching and tempering, to attain full surface hardness.

The surface decarburization is accomplished preferably by introducing into the sintering furnace during the last part of the sintering operation an atmosphere which is strongly decarburizing, but not oxidizing. It is desirable that the carbon gradient between the decarburized layer and the interior of the piece should be as steep as possible; the surface of the piece should be substantially completely decarburized to a small depth for example of the order of about 4 of an inch, and below this the carbon content should rise abruptly to the percentage desired in the final piece.

The invention will be further illustrated in conjunction with the accompanying fiow sheet wherein stage 1 representsthe mixing of iron powder and carbon powder in'suitable proportions; stage 2 represents pouring a charge of the mixed powders into a die; stage 3 represents subjecting the powder to compression and ejecting the formed compact; stage 4 represents sintering the compact to form a composite body; stage 5 represents heat treating of the sintered compact to decarburize its surface; stage 6 represents repressing of the sintered compact; and stage '7 represents heat treating of the repressed compact to restore carbon to the decarburized surface. More specifically and by way of illustration the following eX-- ample describes in detail a manufacturing procedure according to the invention which has proved to be successful and practical.

Powdered electrolytic iron is mixed with 0.6% powdered graphite, 0.63% low carbon ferro-manganese powder, and 1.0% powdered stearic acid. This mixture is compressed in conventional manner into porous compacts under a pressure of 27 tons per square inch. The compacts thus obtained are then pro-heated at 900 F. in a hydrogen atmosphere until the stearic acid is substantially completely eliminated. The compacts are thensintered for 2 hours and 55. minutes at 2000 F. in an atmosphere of cracked ammonia having a dew point of C. and containing 0.36%- methane, this atmosphere being substantially in equilibrium with steel of 0.6% carbon at 2000 F. At the end of this time, with the pieces still in the furnace at 2000 F., the atmosphere is changed to cracked ammonia having 10 C. dew point but with no methane, and the pieces are held atv 2000 F. in this clecarburizing atmosphere for minutes. The pieces are then pulled out into the cooling zone of the furnace to cool.

The above described preheating, sintering' and decarburizing steps are preformedin heat treat ing furnace equipment of conventional type, the furnace being preferably provided with a gas. tight metal mufile for example of nickel chromium alloy and having supply connections and vents to introduce and remove toherein described atmospheres. The muffle preferably extends through the hot zone as well as the cool, water jacketed zone of the furnace, so that the briquettes under treatment can be pulled from the hot into the cold zone, the muffle interior being made accessible while retaining therein the proper atmosphere, by

7 suitable Well known provisions such as a flame curtain.

The above described treatment produces a substantially carbon-free surface layer of ferrite approximately of an inch thick below which the material is pearlite having a carbon content of about 0.6%. The transition from ferrite to 0.6% carbon pearlite is very abrupt.

The pieces are then repressed or coined in conventional manner in a die at a pressure of 90 tons. per square inch. During the repressing operation the metal of the. compact which comes into contact with the surfaces of the die and punches is the soft, non-abrasive iron produced at the surface of the pieces as described above. I

After coining, the pieces are resintered at 2000 F., in furnace equipment'of the above mentioned type. During the first 15 minutes of the sintering operation the sintering atmosphere consists of cracked ammonia having a -10 C. dew point, containing 0.75% of propane. At the sintering temperature of 2000 C., this is a highly carburiz-- ing atmosphere which quickly restores carbon to the surface of the pieces. Thereafter the. atmosphere. is changed to cracked ammonia with a l0 C. dew point, containing 0.36% methane. This atmosphere is. substantially in equilibrium with steel of approximately 0.6% carbon at the sintering temperature of 2000 F. The pieces are held at this temperature in this atmosphere for 45 minutes and are then cooled by withdrawing int the cooling zone of the furnace. The resulting pieces have a substantially uniform carbon content throughout, of approximately 0.6%.

Other ways of producing a desirable decarburized layer at the end of the first sintering'operation are possible. For example, the pieces may be slowly cooled in the same atmosphere which was in equilibrium with the piece at the sintering temperature. As the temperature drops such an atmosphere becomes decarburizing and produces the. desired decarburized layer which may be recarburized after the repressing or coining operation as above described.

According to still another embodiment of the 7 method according to the invention, I may remove the pieces from the sintering furnace while still hot and expose them for a short period to the air which burns out the carbon through a shallow surface layer, together with producing an iron, oxide layer at the surface. Thereupon I put the pieces back into the sintering atmosphere long enoughv to reduce the oxide leaving the briquette with a clean decarburized surface layer. The piece is then promptly cooled.

Although the above recarburiz-ing step by means of a carburizing atmosphere is preferred, it is. also possible to recarburize with more primitive means for example by packing the pieces in carbon. powder and heating the. pile. to the sintering temperature. Although this step does not. lend itself to easy control of the degree of carburization it. is quite satisfactory in certain cases.

It will be understood that the present invention deals not only with bodies which are obtained by pressing a mixture of iron and carbon powders, but that it also contemplates. introducing carbon by initially using a steel powder,

or by adding carbon subsequently to the first pressing, as in a sintering step.

If more than one repressing operation is desired, I may decarburize the surface. in thesintering operation preceding each repressing, and restore the carbon in the final sintering operation, as described above for a single decarburizing and recarburizing operation.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications, and equivalents which fall within the scope of the appended. claims.

I claim:

l. A method of making steel bodies from powdered metal in a manner to minimize. die. Wear comprising the. steps of mixing substantially carbon-free iron powder and free. carbon powder together, compressing the mixture to form a co, herent substantially porous body, sintering the coherent body in an atmosphere substantially in carbon equilibrium with the body at the sintering temperature to cause. the carbon to dissolve in the iron to form a steel body, removing substantially all of the dissolved carbon from the surface region of the steel bod sothat the surface is constituted by a layer of soft substantially carbon-free iron. coining the body, and then restorin carbon to said decarburized surface region of said body to the desired level by sintering the. body in a carburizing atmosphere.

2. In making steel articles from. powdered materials by two pressing operations each followed by sintering so. as to minimize die wear in the second pressing operation, the method comprising the steps of mixing substantially carbon-free iron powder with powdered carbon, compressing the mixed powders to form a coherent compact, sintering the compact at a temperature sufiicient to cause the carbon to dissolve in the iron in an atmosphere which is substantially in equilibrium at the sintering temperature with the steel compact, continuing the sintering in a decarburizing atmosphereto produce a thin substantially carbon-free surface on the compact, repressing the compact at a pressure of at least 60 tons per square inch, and resintering the compact in a carburizing atmosphere to restore carbon to said carbon-free surface.

3. A method of making steel bodies from powdered metal by two pressing operations, each followed by sintering in a manner to minimize die wear during the repressing operation, said method comprising the steps of compressing a powder of iron and carbon to form a coherent substantially porous body, sintering the coherent body in an atmosphere substantially in carbon equilibrium with the body at the sintering temperature to 0 form a steel body, further sintering the body in an atmosphere of reduced carburizing potential to effect removal of substantially all of the carbon from the surface region of the steel body so that the surface is constituted by a layer of soft substantially carbon-free iron, repressing the body, and then restoring carbon to said decarburized surface region of said body to the desired level by sintering the body in a carburizing atmosphere.

RICHARD E. RICE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,452 Clements et a1. Mar. '7, 1944 1,940,294 Calkins Dec. 19, 1933 1,953,925 Boegehold et a1 Apr. 10, 1934 2,175,850 Patterson et a1 Oct. 10, 1939 2,342,799 Goetzel Feb. 29, 1944 

3. A METHOD OF MAKING STEEL BODIES FROM POWDERED METAL BY TWO PRESSING OPERATIONS, EACH FOLLOWED BY SINTERING IN A MANNER TO MINIMIZE DIE WEAR DURING THE REPRESSING OPERATION, SAID METHOD COMPRISING THE STEPS OF COMPRESSING A POWDER OF IRON AND CARBON TO FORM A COHERENT SUBSTANTIALLY POROUS BODY, SINTERING THE COHERENT BODY IN AN ATMOSPHERE SUBSTANTIALLY IN CARBON EQUILIBRIUM WITH THE BODY AT THE SINTERING TEMPERATURE TO FORM A STEEL BODY, FURTHER SINTERING THE BODY IN AN ATMOSPHERE OF REDUCED CARBURIZING POTENTIAL TO EFFECT REMOVAL OF SUBSTANTIALLY ALL OF THE CARBON FROM THE SURFACE REGION OF THE STEEL BODY SO THAT THE SURFACE IS CONSTITUTED BY A LAYER OF SOFT SUBSTANTIALLY CARBON-FREE IRON, REPRESSING THE BODY, 